Physical controls on biogeochemical zonation in the Southern Ocean

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Abstract

The primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilization.

Résumé

La zonation nord-sud de l’Océan Austral est contrainte par le transport, induit par le vent, du Courant Antarctique Circumpolaire (ACC). Ce Courant divise l’Antarctique en 3 zones majeures: la zone subantarctique (SAZ) au nord de l’ACC, la zone de l’ACC proprement dite, la zone située au sud de l’ACC (SACCZ). La zone de l’ACC comprend la zone du Front Polaire (PFZ) et la zone antarctique (AAZ) mais il peut être opportun de la diviser plus finement selon les régions considérées. Pour maintenir l’équilibre géostrophique les isopycnes présentent un mouvement ascensionnel du nord au sud, permettant à des eaux riches en sels nutritifs de parvenir en surface dans la zone sud de l’ACC. De plus les teneurs en silicates dans les eaux de surface s’accroissent vers le sud en raison du mélange diapycnal avec les eaux profondes riches en silicates. Un gradient de silicates est donc observé du sud (eaux riches de la SACCZ) au nord (eaux pauvres de la SAZ). Les silicates peuvent être limitants de la production des diatomées en été. Les nitrates présentent également un gradient décroissant vers le nord mais ne sont limitants qu’en zone subtropicale, au nord de la SAZ. Un élément important du contrôle de la biologie par la physique est la variablité de la stratification de la couche de surface, à origine thermique dominante dans la SAZ ou à origine haline dominante plus au sud par fusion de la glace de mer. Au niveau circumpolaire il en résulte la formation d’un minimum thermique subsuperficiel (typiquement 2°C) et une subduction des Eaux Antarctiques Intermédiaires caractérisées par un minimum de salinité, souvent associés à des transports importants et à des jets frontaux. Le transport dynamique latitudinal de l’ACC est contraint en bandes étroites dont le nombre et la position dépend de la topographie. A proprement parler c’est le transport total de l’ACC ainsi que les propriétés scalaires de salinité et de température qui sont circumpolaires et non les fronts. Bien qu’il s’agisse en principe de systèmes HNLC (high nutrient low chlorophyll) des consommations intenses de silicates et de nitrates ont eté reportées dans toutes les zones (SAZ, PFZ, AAZ). Des floraisons phytoplanctoniques intenses couvrant de large zones (échelle de 400 km) dans la PFZ et dans l’AAZ sont fréquentes à proximité des plateaux sous-marins, ce qui pourrait provenir d’une fertilisation locale en fer, à partir des sédiments.

Introduction

The dominant physical control on biogeochemical distributions in the Southern Ocean is the banded structure of the Antarctic Circumpolar Current (ACC), descriptions of which go back to Deacon (1933) and beyond. Deacon (1982) summarized his 50 years of experience of physical and biological zonation in a paper still often referenced, starting with the role of the winds in driving convergences and divergences in the surface layer. More recently, physicists (e.g., Whitworth and Nowlin, 1987; Orsi et al., 1995; Belkin and Gordon, 1996) have described the fronts of the Southern Ocean, seeking to define their salinity and temperature properties and asking whether they are circumpolar in extent. In this paper we shall review both the role of the winds and the question of what features are circumpolar in order to examine chemical and biological zonation in relation to the physics. We need to start by summarizing definitions of fronts and zones in common use before developing our own definitions, which differ subtly but significantly from those in the literature.

Whitworth and Nowlin (1987) describe three main fronts, from north to south, the Subtropical Front (STF), Subantarctic Front (SAF) and Polar Front (PF, sometimes referred to as the APF, for Antarctic Polar Front). Orsi et al. (1995) add a fourth circumpolar feature, the Southern Boundary of the ACC (herein abbreviated to SB). Already these terms differ from those used by Deacon (1982), who referred to Subtropical Convergence (now STF), Antarctic Convergence (now PF), and Antarctic Divergence (which bears some relation to the SB), which are terms still commonly found in biological literature. Deacon's terminology related to features in the global scale wind field and to convergences and divergences in the Ekman drifts in the upper ocean that would result from them. However, the wind-driven convergences and divergences cover much wider bands of latitude than the sharp fronts of the Southern Ocean, so it is preferable to use the newer terminology that does not mention nor assume convergence or divergence.

Indeed, Ekman drifts are a secondary effect of the wind forcing. The primary effect of the strong winds of the Roaring Forties is to drive the ACC itself, which, in the circumpolar Southern Ocean uniquely, results in transports of 130–140 Sv (1 Sv=106 m3 s−1) (Nowlin and Klinck, 1986). In turn, for these currents to be in geostrophic balance, isopycnals must slope up to the south, which tends to occur in narrow frontal bands (a natural consequence of the small scale, tens of kilometres, of the Rossby radius of deformation) whose meandering courses are strongly steered by bathymetry. At each front, the sloping isopycnals expose different water masses to the surface layer, or remove them from it, resulting in different stratification and surface zonation with its biological consequences.

We start by defining the STF, SAF, PF and SB (Table 1), seeking correspondence with two of our own data sets, which will highlight some shortcomings of these definitions. The data sets we shall repeatedly refer to are two, quasi-meridional sections (Fig. 1). The first (Fig. 2, Fig. 3), along 40–42°E, was part of the SouthWest Indian ocean EXperiment (SWINDEX) hydrographic survey described by Pollard and Read (2001), a UK contribution to the World Ocean Circulation Experiment. The second (Fig. 4), between 2°W and 13°E, described by Read et al. (2002), was collected at the start of a German Southern Ocean JGOFS cruise. For brevity, we shall refer to these two sections as 40E and 5E (without the ° symbol).

The STF is the boundary between Subtropical Surface Water and cooler, fresher Subantarctic Surface Water. We need add little to the original description by Deacon (1937) and numerous authors since then, summarized by Belkin and Gordon (1996). Belkin and Gordon (1996) note that there may be several frontal jets (concentrated currents) in a Subtropical Frontal Zone, which they define as bounded by North and South STFs. The possibility of several jets in a frontal zone is an important concept, to which we shall return. In our examples, the Southern STF is clearly seen along 40E between 41° and 42°S in Fig. 2, Fig. 3. Note that it is not a full-depth feature, as the isopycnals (Fig. 2c) become horizontal below 2000 m. On 5E (Fig. 4) the large, sharp (across barely 10 km) temperature and salinity changes mark the STF. Unfortunately the STF was crossed very shortly after SeaSoar deployment at the start of the cruise and collection of surface nutrients had not yet begun (Fig. 4f).

North of the SAF there is a subsurface salinity minimum associated with subducting Antarctic Intermediate Water (AAIW). South of the SAF the lowest salinity water is in the surface layer. This change in vertical structure is the major identifier of the SAF (Whitworth and Nowlin, 1987). Put another way, north of the SAF salinity can decrease downwards because there is sufficient temperature stratification in the upper layers to compensate for a statically unstable salinity gradient. Thus temperature dominates the stratification north of the SAF, whereas to the south salinity and temperature contribute about equally.

The SAF is the northernmost frontal jet that passes through Drake Passage (Sievers and Nowlin, 1984) and is generally regarded as circumpolar in extent (Nowlin and Klinck, 1986). However, it may be merged with the PF (Read and Pollard, 1993) and may not be meaningfully defined in the vicinity of the Del Caño Rise (Pollard and Read, 2001). Thus, in Fig. 2, although the position at which a subsurface salinity minimum appears is easy to identify and label the SAF (Fig. 2b), the lack of sloping isopycnals (Fig. 2c) means that there is no current jet associated with it. Similarly, Read et al. (2002) defined the SAF at 48°S along 5E (Fig. 4) primarily by reference to the changes in stratification, as currents were relatively weak. Orsi et al. (1995) use the 0.9 dyn m dynamic height contour to define the northern boundary of the ACC, which lies not far to the north of the SAF at most longitudes, with the greatest discrepancies in the southwest Atlantic sector.

The PF is most frequently identified by the northernmost extent of the 2°C subsurface temperature minimum (Belkin and Gordon, 1996). If we ask how a temperature minimum with depth can exist, that is, of course, because the statically unstable temperature gradient is compensated for by the very fresh surface salinities; hence salinity dominates the stratification everywhere south of the PF. This change in the balance of stratification is the significant property change that is marked by the PF. If we ask further, why 2°C, we realize that the value of the temperature minimum is determined by the underlying Upper Circumpolar Deep Water (UCDW), which has a temperature maximum a little greater than 2°C.

The 2°C temperature minimum is not everywhere associated with a major current jet, for example south of Crozet (50°E), as discussed by Pollard and Read (2001). They showed that the ACC fragments as it crosses the Southwest Indian Ridge, resulting in two current jets crossing 40°E (Fig. 2), which we have labelled the PF and SPF (Southern PF) in order to distinguish them (see also Fig. 5). Rintoul and Bullister (1999) also reported two branches of the PF near 140°E. Following the 2°C definition, the jet at 48.5°S in Fig. 2 is the PF, but property changes across the southern jet (Southern Polar Front (SPF)) at 50.5°S are more pronounced. Note particularly the large horizontal changes in silicate (Fig. 2d) and the T/S and silicate changes on the 28.0 neutral density marked on each subplot of Fig. 2, showing a water mass boundary at 50.5°S. Similarly along 5E (Fig. 4), while the 2°C temperature minimum ends at 49°S (Fig. 4a), more significant changes, both physical and biological, were found at the front at 51.5°S (Pollard et al., 2002), which Read et al. (2002) called the “surface expression of the PF” following several previous authors. Thus, while 2°C marks the northernmost position of the PF, more significant frontal jets may be associated with colder subsurface temperature minima, 0.5–1.5°C along both 40E (Fig. 3a) and 5E (Fig. 4a). Because a significant branch of the ACC (i.e. current jet, so baroclinic front) south of the 2°C temperature minimum is found at many latitudes, we shall label it the SPF to distinguish it from the PF. We do not claim, however, that the SPF is circumpolar, as Read et al. (1995) did.

South of the PF several fronts have been described and there has been some confusion in the literature about which are significant and which circumpolar. Deacon (1933) himself described surface and subsurface expressions of the PF, as did Sievers and Nowlin (1984) at Drake Passage, and Lutjeharms and Valentine (1984) found them up to 300 km apart. Note that the subsurface expression of the PF is the northernmost feature, and corresponds to the definition of the PF given above. Read et al. (1995) described the Southern Polar Front found in the Bellingshausen Sea but which they could identify at least as far as the Greenwich Meridian and Orsi et al. (1995) defined a “southern ACC front”.

In our view, however, the most important feature identified by Orsi et al. (1995) is not the southern ACC front but the SB of the ACC, which Orsi et al. (1995) could clearly identify in numerous sections around the Southern Ocean as the southern terminus of UCDW. In some sectors the SB equates with the Continental Water Boundary, with continental waters to the south. In others it equates well with the northern boundaries of the Weddell and Ross gyres. It also lies close to the 0.35 dyn m dynamic height contour, which Orsi et al. (1995) used as the southernmost dynamic height contour to pass through Drake Passage. At Drake Passage Orsi et al. (1995) found that the SB lies a little to the south of the southern ACC frontal jet, and they concentrated on the latter, arguing that the southern ACC front is circumpolar. In our view it is the SB that is the well-defined circumpolar feature because, at all longitudes, there will be some latitude at which the UCDW outcrops and ends. Several frontal jets lie between the SB and the PF, none of which really merits naming individually, as they vary with longitude depending on the underlying bathymetry. However, we have made an exception for the SPF, because it can be more significant than the PF at the longitudes where it occurs.

Our section 40E did not go far enough south to reach the SB, but along 5E the position of the SB can be inferred to lie where the 1.5°C isotherm (Fig. 4a) suddenly plunges down past 350 m near 56°S. Surface silicate changes suddenly at that latitude (Fig. 4f) and isopycnals slope sharply up to the south (Read et al., 2002) inducing a strong surface current. Using the data of Whitworth and Nowlin (1987), Orsi et al. (1995) placed the SB at 56°S at the Greenwich Meridian, in nearly exact agreement with our section 5E, which crossed the Greenwich Meridian at 56°S.

The difficulty we have had in associating a frontal jet and only one frontal jet with each of the fronts STF, SAF, PF or SB indicates that we should consider a different approach to defining the circumpolar features of the Southern Ocean. Our fundamental conclusion is that it is not the strong current jets that are circumpolar, but latitudinal changes in structure that are induced by the changing contributions of temperature and salinity to the stratification with latitude. The winds generate 130–140 Sv of transport. How that transport is partitioned between fronts is determined by the bathymetry and will vary with longitude, as is apparent in Fig. 1 but was already well shown in current amplitudes from one of the first eddy-permitting Southern Ocean models (Webb et al., 1991). There is no physical reason why there should be, say, three circumpolar fronts (the SAF, PF and southern ACC front) carrying most of the ACC transport. Thus it is not the frontal jets that are circumpolar in extent. The ACC transport can be confined to a few jets or fragmented into many, depending on how the bathymetry channels the circulation.

What must be circumpolar, by simple mass balance, is the total transport of the ACC. For that transport to be in geostrophic balance, isopycnals must slope up to the south and it is easy to show that, for 140 Sv total transport, isopycnals must shallow by about 1000 m. This rise is the same whether the transport is spread across a wide or narrow band of latitudes. Thus, for example, the UCDW, found at a depth of 1000–1500 m north of the ACC, must rise by about 1000 m across the ACC, so that it outcrops at the southern edge of the ACC, thus forming the SB, which must therefore be circumpolar. In general, it is the outcropping of different isopycnals and the water masses associated with them that results in the physical zonation of the Southern Ocean. There is one more important point to be made. The fact that all isopycnals rise towards the pole does not tell us anything about convergence or divergence of the upper water masses. The zonation is not primarily a consequence of wind-driven convergence or divergence of the surface layer, but of the sloping isopycnals that must be present to ensure geostrophic balance of the ACC.

We therefore shall define four zones without reference to the frontal jets, but in line with the definitions of Nowlin and Klinck (1986), namely the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), Antarctic Zone (AAZ) and the Zone south of the ACC (SACCZ). Table 1 summarizes our definitions and links them to the frontal definitions given above. A comprehensive summary of frontal definitions is given by Belkin and Gordon (1996). Only a few of these have been extracted into Table 1, chosen to show that fronts have often been defined by the properties of the adjacent zone rather than by the properties of the front itself. The definition of the SB is taken from Orsi et al. (1995).

The SAZ is the northernmost zone of the Southern Ocean and is where surface temperatures are large enough that temperature stratification dominates over salinity stratification. Thus it is possible for salinity to decrease downwards, so the SAZ is defined by a marked subsurface salinity minimum associated with the relatively fresh AAIW. While the SAZ does not carry the ACC transport, it contains much fresh Subantarctic Surface Water that has been carried northwards from the PFZ in the surface layer by wind-driven Ekman transport.

In the PFZ salinity is as important as temperature in contributing to the stratification. Thus, while a weak subsurface salinity minimum or temperature maximum may sometimes be present, in general salinity increases downwards with Subantarctic Surface Water in the surface layer and temperature decreases downwards. Thus the absence of both a marked subsurface AAIW salinity minimum and of a marked shallow subsurface temperature minimum of 2°C or less defines the PFZ.

In the AAZ salinity is more important than temperature in controlling the stratification of the upper ocean. Thus temperature can increase downwards towards the temperature maximum of UCDW, and the induced near-surface temperature minimum (⩽2°C) is the defining characteristic of the AAZ, coupled with indicators (such as a nitrate maximum) that the underlying warmer water is UCDW.

The most poleward zone of the Southern Ocean is the SACCZ, lying south of the outcrop or southern terminus of UCDW (the SB by Orsi et al's (1995) definition). The best way to distinguish the SACCZ from the AAZ is that the subsurface nitrate maximum of UCDW is absent. However, lacking nutrient data, an equivalent indicator is that the subsurface temperature maximum is <1.6°C.

Clearly these definitions are closely related to the previous definitions of the fronts, given by Whitworth and Nowlin (1987) as the location of the rapid descent of the salinity minimum for the SAF and the location of the rapid descent of the 2°C temperature minimum for the PF, quoting Gordon (1967). Our definitions of zones do not depend on the existence of the fronts, however, and can be used to determine the zone whether or not the positions of the fronts are known. However, strong frontal jets must be places where isopycnals slope most steeply. Thus it is highly likely that, say, the descent of the AAIW salinity minimum will be associated with a current jet that we can label the SAF. But, in the absence of a concentrated current (e.g. Fig. 2b), the AAIW salinity minimum will still descend, marking transition from the PFZ to the SAZ, even though the descent will not be “rapid”.

Section snippets

Physical zonation

While we have defined four zones to match previous terminology, it is important to recognize that it is no more valid to say that there are four biophysical zones than it is to say that there are three fronts. We could define zonation based on any number of features that are circumpolar, for example 2°C, 1.8°C and 1.6°C temperature minima. What is more useful is to note that, at any particular longitude, the bathymetry defines the major pathways of the ACC, i.e. the concentrated currents or

Conclusions

In this paper we have sought to revise our paradigm for the physical structure of the Southern Ocean and its biological consequences. The main circumpolar feature is the wind-driven transport of the ACC. The ACC transport tends to be concentrated in frontal jets, the number, strength and latitudinal locations of which are determined by the bathymetry and so vary with longitude. Because the frontal jets can merge or fragment along their paths, it is misleading to think of any of them as being

Acknowledgements

We are particularly grateful to Victor Smetacek, Uli Bathmann, Paul Tréguer and Michel Denis who made it possible for us to participate in German and French JGOFS cruises. OCCAM data were kindly supplied by Andrew Coward. The authors would like to thank the SeaWiFS Project (Code 970.2) and the Distributed Active Archive Center (Code 902) at the Goddard Space Flight Center, Greenbelt, MD 20771, for the production and distribution of these data, respectively. These activities are sponsored by

References (33)

  • S.J. Weeks et al.

    Phytoplankton pigment distribution and frontal structure in the subtropical convergence region south of Africa

    Deep-Sea Research

    (1996)
  • I.M. Belkin et al.

    Southern Ocean fronts from the Greenwich Meridian to Tasmania

    Journal of Geophysical Research

    (1996)
  • P.W. Boyd et al.

    A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

    Nature

    (2000)
  • R.M. Crawford

    The role of sex in the sedimentation of a marine diatom bloom

    Limnology and Oceanography

    (1995)
  • G.E.R. Deacon

    A general account of the hydrology of the South Atlantic Ocean

    Discovery Reports

    (1933)
  • Deacon, G.E.R., 1937. The hydrology of the Southern Ocean. Discovery Reports 15,...
  • Cited by (210)

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